A survey is made on several traditional topologies of fly-by-light(FBL) network. Based on the comparison of their reliability and real-time performance, a mesh optical switching network topology with 256 nodes is chosen. The redundancy topology gives the ability to sustain 3 link faults, which assures the reliability of network. The seriousness of the network failure can be measured by the number of link faults. And when several link faults occur at the same time, it must also be measured by the contiguity degree among the faults. A simulation is taken to test the end-to-end latency under all network failure situations. The simulation results show that even when some serious fault occurs, the number of hops of the end-to-end route increases slightly. So it still meets the real-time communication requirements.